The findings, which come from a study of more than 227,000 women diagnosed with breast cancer, are somewhat surprising because experts have long put breast cancer on a list of risk factors for colon and rectal cancer, right alongside known risk factors like having a first-degree relative with colon or rectal cancer and having a history of polyps.

"In this analysis, clearly breastcancer isn't a risk factor -- and if anything, there is the suggestion that there is decreased risk in certain subgroups," says Craig Newschaffer, PhD, of Johns Hopkins School of Hygiene and Public Health in Baltimore. He is the lead author of the study, which appears in the March 17 issue of the British medical journal The Lancet.

Newschaffer and colleagues found that women diagnosed with breastcancer between 1974 and 1995 had a 5% lower risk of colon cancer and a 13% lower risk of rectal cancer when compared with women in the general population who had never had breast cancer.

While the researchers don't know for sure how breast cancer might protect against colon and rectal cancer, they say it could have to do with the fact that women who have had breast cancer are monitored more closely for other problems. Another possibility is that after having survived breast cancer, women may make healthy lifestyle choices like eating better and exercising -- which are just the things that can protect from colon and rectal cancer later in life.

Subscribing to this line of thought is Theodora Ross, MD, PhD, a professor of medicine at the University of Michigan Comprehensive Cancer Center in Ann Arbor, who assessed the study for WebMD.

"These patients are doing something right," she says, but adds that women with family histories of colon or rectal cancer and those with familial breast cancer -- which involves genetic mutations -- are probably at higher risk than other breast cancer patients for colon and rectal cancer and should be monitored closely.